Now that a reproducing population of zebra mussels has apparently found a home in Lake Ray Roberts, state and federal officials, local water utilities and power plant operators are preparing for the worst.

Experts say the mussels will probably expand south through the Trinity River system, clogging water treatment plant intake pipes along the way and raising costs for customers, with little to slow their explosive growth. The only question is how far they’ll get.

The invasive mollusks have spread across 29 states since they were discovered in the U.S. in 1988, and they’ve proved tougher than anything nature throws at them.

But they’ve never dealt with the searing Texas heat. And there is some small level of hope in that.

“When the mussels first hit the U.S. in the 1980s, the information we had from Europe, where they’ve had them for 100 years, was that they didn’t survive long-term exposure past 28 degrees” Celsius, about 82.4 degrees Fahrenheit, said Robert McMahon, a biologist at the University of Texas at Arlington who specializes in freshwater and marine invertebrates.

“If you looked at a water atlas of the U.S., you could pretty much draw a line [where mussels could be expected to survive] through Lake Texoma, but nothing farther south in Texas,” he said.

So far, the mussels seem to have adapted to temperatures far warmer than their original homes in the Balkans, Poland and the former Soviet Union.

McMahon suggested that at least a portion of the population survived the summer heat in Kansas and Oklahoma, and their descendants might be more heat-tolerant than expected.

“Certainly we’ve seen living larvae in Lake Texoma” at 86 degrees, he said.

The mussels’ appearance in Lake Ray Roberts, north of Denton, is a first in the Trinity River Basin. And the Trinity is the lifeline for the most populous swath of Texas, providing water from Denton to Dallas to Houston.

“This is a big concern,” McMahon said, “but you can’t just say everything will get infested. We just don’t know.”

His hope is that lake temperatures in North Texas are at the edge of mussel mortality.

“We think if you get an average temperature of 32 Celsius — that’s about 90 degrees — they might not survive,” McMahon said, and that might stall their progress south.

But the more immediate concern is keeping the mussels from spreading from Lake Ray Roberts to Lake Lewisville and potentially all the way down the Trinity to Lake Livingston, north of Houston.

That’s almost impossible. Any release of water from one lake to another could carry mussel larvae downstream. The U.S. Army Corps of Engineers, which is responsible for flood control, regularly opens the gates at North Texas lakes after heavy rains to ease flooding potential.

And it doesn’t take that many of the tiny mussels — mature adults measure an inch to 1½ inches long — to completely transform a lake. Their reproductive abilities are breathtaking.

“There are males and females, and they release sperm and eggs into the water,” McMahon said. “Females can release a couple of hundred thousand eggs in a breeding system. So if you have 25,000 females, well, do the math.

“The mussels get into piping and pile up on each other four, five inches thick,” McMahon said. “That can cause big problems for power plants and big problems for water systems.”

Power plant operators dealt with a similar problem in the 1970s from a species called the Asian clam and developed treatments, he said. But public water systems can’t use chemicals that might kill the mussels.

Dallas Water Utilities relies on Lake Ray Roberts and its southern neighbor, Lake Lewisville, for part of its water supply, and that won’t change, the city said.

“The presence of zebra mussels does not impact the city’s ability to use its water supply in the [Corps of Engineers] owned and operated Lake Ray Roberts,” the city’s statement said. “What it can mean is that the adult mussel will attach to water intakes and can clog water intakes, which then have to be cleaned.

“This would add additional operations costs which are not currently in our budgets.”

Jim Coulter, assistant Denton city manager, said Denton Water Utilities officials will initiate a monitoring program to detect the presence of the mussel at the facilities.

“We do not anticipate having to provide treatment in Year One, but we will be ready should this need arise.”

Coulter said that depending on the severity of the infestation and the size of the facility, repair costs could range from several hundred thousand dollars to $1 million per year.

Barry Van Zee, inland fisheries regional director for Texas Parks and Wildlife Department, said he knew of only one case in which a mussel population was wiped out — in a small rock quarry covering barely an acre that was heavily dosed with potassium chloride.

Parks and Wildlife’s efforts to kill a population of mussels in Sister Grove Creek in Collin County, which feeds into Lavon Lake, was “partly successful,” he said. “We had mortality, but it was not 100 percent.”

And that was in a creek narrow enough to jump across in spots, he said.

“They tried an eradication plan in Kansas — they drew the reservoir down and killed everything,” he said. “But there were still little puddles and pools left on the bottom,” and with all the predators gone, “the zebra mussels came back with a vengeance.”

The consensus now is that once zebra mussels get established in a water system, “there is no way of getting rid of them,” Van Zee said.

There are ducks and a couple of kinds of fish that eat them, he said, but the mussels are so prolific “that you can’t keep up.”

“In Oklahoma, they’ve found where blue catfish were feeding on them, and when they’d passed through the digestive system, about half of them still survived,” he said.

The bottom line, said McMahon, is the mussels are likely to move downstream and to establish themselves in other lakes. And that means higher costs for the utilities that take water from those lakes.

“Fouling [water intakes] gets expensive, and in the end it all comes out of our pockets,” McMahon said.

It’s difficult to persuade the public to provide money for better inspections of ships to make sure their ballast water — which probably brought the mussels to the U.S. in the first place — is clean.

“But the cost of doing that is pretty small compared to having to deal with them once they’re here,” McMahon said.

“It’s like having insurance,” he said. “If you don’t, the ultimate costs are many, many times higher.”